As shown in FIG. 5, a typical rail 42 is fixed to a wood block 41 and is fixed to a post 4 by a bolt 43 and a washer 44. The wood block 41 is parallelepiped and includes six flat outer surfaces so that the contact area between the post 4 and the block 41 and between the block 41 and the rail 42 is very small so that the rail 42 can not be stably retained in place.